The present invention relates generally to automated bonding, and more particularly to a method for accurately producing a ball bump on a surface which is used to provide a conductive bond with another surface.
Ball bumps have specific utility in microelectronic applications for conductively bonding one surface of a workpiece to another surface of a workpiece at predetermined bond sites on the surfaces. The workpiece is typically a wafer or a substrate having a planar surface and the bond sites are precisely defined areas on the surfaces of the wafer or substrate. The wafer and/or substrate is ultimately used in the fabrication of a microelectronic device after further processing. For example, a wafer typically consists of a plurality of interconnected die sharing a continuous common surface. Each die is a tiny semiconductor components, such as a diode, transistor or integrated circuit, which has a bond site on the continuous common surface. A ball bump is produced at every bond site on the wafer and the wafer is then cut into individual die. Thereafter, each individual die is bonded to a corresponding bond site on a substrate by means of the ball bump which is bonded to the bond site on the individual die. The substrate is typically a planar structure substantially larger than the individual die such as a printed circuit or an integrated circuit package.
Ball bumps are commonly formed on bond sites by a machine automated technique, wherein the end of a very fine electrically-conductive metal wire is played out through the end of a capillary, which serves as a wire guide. The end of the wire is flamed to melt the wire and the resulting molten metal beads up to create a ball beneath the end of the capillary, which remains attached to the wire when cooled. The end of the capillary is aligned over the desired bond site on a surface and the end of the capillary is lowered down toward the bond site to place the ball on the bond site. The end of the capillary is lowered further still, compressing the ball against the bond site. The compression caused by the capillary in conjunction with energy applied to the ball from an external source bonds the ball to the bond site while flattening the ball to reduce its height and increasing its areal spread. Once the ball is flattened, the end of the capillary is drawn away from the bond site to break the connection between the wire and the ball, leaving a newly formed ball bump on the bond site.
The ball bump may then be used to bond the first surface, to which the ball bump has been bonded, to a second surface. Bonding of the two surfaces is effected by aligning the ball bump on the first surface with a bond site on the second surface and forcing the ball bump against the bond site while simultaneously heating or otherwise applying energy to the ball bump. A thermal or mechanical compression bond forms between the ball bump and second surface, thereby bonding the first and second surfaces together. Because the ball bump is formed from an electrically conductive metal, the ball bump functions not only as an adhesive for the bond between the two surfaces, but as an electrical conductor between the two surfaces.
Accurate shaping of the ball bump at the bond site of the first surface is critical to the quality of the resulting bond between the first and second surfaces. If the ball bump spills outside the defined boundaries of the desired bond sites on either surface, the ball bump may encroach into operating areas of the surfaces and diminish the overall operation of the microelectronic device in which the surfaces are employed. Moreover, if the top surface of the ball bump is uneven or too high upon its formation, the non-conforming character of the top surface may disrupt the ability to properly bond the first and second surfaces together. Accordingly, tolerances for the ball bump in all three dimensions X, Y, and Z are very strict for certain microelectronic applications, being on the order of about 1 to 5 microns.
It has been found that prior art methods for producing ball bumps are oftentimes unable to satisfy the strict performance requirements of many microelectronics applications. In high-speed production runs, practical considerations limit the period of time allotted to produce each ball bump, which compounds the difficulty in meeting the required dimensional tolerances for the ball bumps. For example, the step of breaking the attachment between the wire and the ball at high speed to produce the ball bump frequently leaves a residual xe2x80x9ctailxe2x80x9d of wire extending from the top surface of the ball bump, which may be disruptive to subsequent bonding of the ball bump to another surface. As such, the present invention recognizes a need for a method of accurately and repetitively producing tailless ball bumps at a high rate of speed.
Accordingly, it is an object of the present invention to provide a method of producing a tailless ball bump. More particularly, if is an object of the present invention to provide such a method which is fully automated and computer controlled. It is another object of the present invention to provide such a method which can be accurately repeated many times over. It is still another object of the present invention to provide such a method which can be performed at a high rate of speed. It is a further object of the present invention to provide such a method which can produce a plurality of dimensionally consistent ball bumps, and more particularly a plurality of ball bumps having a uniform desirable height, at a high rate of speed. These objects and others are accomplished in accordance with the invention described hereafter.
The present invention is a method of producing a tailless ball bump on a workpiece. A selectively positionable capillary is provided having a passageway therethrough and having an opening from the passageway out of the capillary. A wire is positioned in the passageway and an end of the wire is extended from the opening. A ball is positioned on the end of the wire which has a ball height. The capillary is positioned proximal to a working surface on a workpiece and the ball is positioned on a bond site on the working surface. The capillary applies a force against the ball in a direction of the bond site to transform the ball to a ball bump. The ball bump has a top surface and a ball bump height above the working surface, which is less than the ball height. The ball bump becomes bonded to the bond site upon formation. The capillary is displaced thereafter in a first direction away from the ball bump to detach the wire from the ball bump. A residual wire tail, which extends away from the top surface of the ball bump, is formed when the wire is detached from the ball bump. The capillary is displaced in a second direction across the top of the ball bump to engage the residual wire tail, return the residual wire tail to the top surface of the ball bump, and substantially flatten the residual wire tail against the top surface of the ball bump. The second direction is preferably a horizontal direction substantially opposite the first direction. The above-recited steps may be repeated as often as desired to form additional ball bumps on separate bond sites, which are on the same workpiece or on different workpieces. The above-recited method may further include the step of applying energy to the ball, preferably in the form of ultrasonic energy and/or heat energy, while applying the force to the ball.
In accordance with another embodiment, the method of present invention provides a bond head having a selectively positionable capillary and wire clamp, which cooperatively produce a tailless ball bump. The wire clamp is selectively positionable in an open position or a closed position and is mounted on the bond head above the capillary. The capillary is positioned along a vertical linear axis at a vertical linear point and along a vertical rotary axis at a vertical rotary point. A bond site on a working surface of the workpiece is positioned along a horizontal linear axis at a horizontal bond point. A wire extends from the capillary and wire clamp and an end of the wire is positioned beneath the capillary. A ball is suspended from the end of the wire.
To form a ball bump, the wire clamp is initially maintained in the open position enabling displacement of the wire relative to the wire clamp. The capillary is displaced along the vertical linear axis to a vertical linear contact point beneath the vertical linear point, thereby positioning the ball on the bond site. The capillary applies a force along the vertical rotary axis against the ball in a direction of the bond site and energy is transmitted through the capillary to the ball. The action of the capillary deforms the ball and bonds the ball to the bond site. Deformation of the ball causes radial displacement of the capillary along the vertical rotary axis from the vertical rotary point to a vertical rotary deformation point. The sum of these effects forms the ball bump, although it still has the wire attached to and extending from its surface.
To detach the wire, the capillary is radially displaced along the vertical rotary axis which withdraws the force from against the surface of the ball bump. The wire clamp is repositioned from the open position to the closed position to releasably fix the wire relative to the wire clamp. The capillary is displaced along the horizontal linear axis to a horizontal wire shear point away from the horizontal bond point, thereby detaching the wire from the ball bump. Detaching the wire forms a residual wire tail which extends from the surface of the ball bump. To remove the residual wire tail, the capillary is displaced along the horizontal linear axis away from the horizontal wire shear point across the surface of the ball bump to a horizontal tail removal point on another side of the horizontal bond point from the horizontal wire shear point. As the capillary is displaced, it engages the residual wire tail and returns the residual wire tail to the surface of the ball bump.
The present invention will be further understood from the drawings and the following detailed description.